[RK3288][Android6.0] WiFi之PNO功能了解

Platform: Rockchip
OS: Android 6.0
Kernel: 3.10.92

概念：
PNO 即Preferred Network Offload，用于系统在休眠的时候连接WiFi
此功能是在Android3.1加入的

缺陷：
在使用PNO时，有潜在泄露个人隐私的风险。这里没明白，意思是说PNO会发送之前的连接过的网络给AP，然后hacker截取？

代码：
当屏幕状态有变化的时候，会调用handleScreenStateChanged(),如果pno功能有enable，就会配置pno,然后再scan.

private void handleScreenStateChanged(boolean screenOn)
{//亮屏的情况if (screenOn) {enableBackgroundScan(false);setScanAlarm(false);clearBlacklist();fullBandConnectedTimeIntervalMilli= mWifiConfigStore.wifiAssociatedShortScanIntervalMilli.get();// In either Disconnectedstate or ConnectedState,// start the scan alarm so as to enable autojoin//如果当前是连接着的if (getCurrentState() == mConnectedState&& allowFullBandScanAndAssociated()) {//PNO功能是否开启了if (useHalBasedAutoJoinOffload()) {//使用PNO来扫描startGScanConnectedModeOffload("screenOnConnected");} else {// Scan after 500msstartDelayedScan(500, null, null);}} else if (getCurrentState() == mDisconnectedState) {if (useHalBasedAutoJoinOffload()) {//调用未连接的PNO扫描startGScanDisconnectedModeOffload("screenOnDisconnected");} else {// Scan after 500msstartDelayedScan(500, null, null);}}//关屏休眠时} else {//当处于未连接状态时if (getCurrentState() == mDisconnectedState) {// Screen Off and Disconnected and chipset doesn't support scan offload//              => start scan alarm// Screen Off and Disconnected and chipset does support scan offload//              => will use scan offload (i.e. background scan)//如果不支持PNO，那么就用alarm做定期扫描if (useHalBasedAutoJoinOffload()) {startGScanDisconnectedModeOffload("screenOffDisconnected");} else {if (!mBackgroundScanSupported) {setScanAlarm(true);} else {if (!mIsScanOngoing) {enableBackgroundScan(true);}}}} else {enableBackgroundScan(false);if (useHalBasedAutoJoinOffload()) {// don't try stop Gscan if it is not enabledstopGScan("ScreenOffStop(enableBackground=" + mLegacyPnoEnabled + ") ");}}}
}

private boolean startGScanConnectedModeOffload(String reason)
{if (!useHalBasedAutoJoinOffload()) return false;//获取pno列表List<WifiNative.WifiPnoNetwork> llist= mWifiAutoJoinController.getPnoList(getCurrentWifiConfiguration());// first program the network we want to look for thru the pno APIWifiNative.WifiPnoNetwork list[]= (WifiNative.WifiPnoNetwork[]) llist.toArray(new WifiNative.WifiPnoNetwork[0]);//设置pnoif (!WifiNative.setPnoList(list, WifiStateMachine.this)) {Log.e(TAG, "Failed to set pno, length = " + list.length);return false;}
}

pno list的创建在buildPnoList():

private void buildPnoList() {mCachedPnoList = new ArrayList<WifiNative.WifiPnoNetwork>();ArrayList<WifiConfiguration> sortedWifiConfigurations= new ArrayList<WifiConfiguration>(getConfiguredNetworks());Log.e(TAG, "buildPnoList sortedWifiConfigurations size " + sortedWifiConfigurations.size());if (sortedWifiConfigurations.size() != 0) {// Sort by descending priorityCollections.sort(sortedWifiConfigurations, new Comparator<WifiConfiguration>() {public int compare(WifiConfiguration a, WifiConfiguration b) {return a.priority - b.priority;}});}for (WifiConfiguration config : sortedWifiConfigurations) {// Initialize the RSSI threshold with sane value:// Use the 2.4GHz threshold since most WifiConfigurations are dual bands// There is very little penalty with triggering too soon, i.e. if PNO finds a network// that has an RSSI too low for us to attempt joining it.int threshold = thresholdInitialAutoJoinAttemptMin24RSSI.get();Log.e(TAG, "found sortedWifiConfigurations : " + config.configKey());WifiNative.WifiPnoNetwork network = mWifiNative.new WifiPnoNetwork(config, threshold);mCachedPnoList.add(network);}
}

是否使用pno依赖于如下三个变量判断
1. Settings里有没有打开
2. 驱动是否支持
3. HAL层是否打开

boolean useHalBasedAutoJoinOffload() {// all three settings need to be true:// - developper settings switch// - driver support// - config optionreturn mHalBasedPnoEnableInDevSettings&& mHalBasedPnoDriverSupported&& mWifiConfigStore.enableHalBasedPno.get();
}

rk3288默认是关闭的，可以从log中直接看到：
WifiStateMachine: Enter ConnectedState mScreenOn=true scanperiod=20000 useGscan=false/false mHalBasedPnoEnableInDevSettings false

参考：
How to Lock Down Your Android Wi-Fi Settings to Improve Privacy
Honeycomb MR1
Android bug leaks your location via WiFi

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